Kai: Grounding is through a conductor capable of safely carrying the worst case fault current from the associated protection device (fuse or circuit breaker) thus for the 400 amp main the ground conductor must be capable of surviving a direct short -perhaps 20,000+ amps long enough to trip the breaker. all sub circuit grounds are thus the same size or smaller depending on the current rating of the circuit. They are NEVER to be used as deliberate current carriers as their sole function is to provide safty in the event of a short from a power conductor to exposed metal. Because ,in a properly configured system there is no current in the ground conductors the exposed metal parts will be at the same potential as the lone ground point for the system.
It is the responsibility of the system designer to insure that the ground grid - the mechanical connection to earth is large enough to maintain very low resistance for the expected life of the instalation across all anticipated ground resistance conditions.
Normaly a service drop's area is small enough and the ground connection large enough that the potential from earth to exposed metal is less than 3 volts rms - here we are not discussing hazardous areas as they naturaly are subject to much more stringent criteria.

Of course lightning strikes in the area can momentarily destroy these conditions and at the same time sensitive equiptment and unlucky people.

now concider a ground loop say through the ground on your scope probe -if you are lucky it manifests as ground noise, if you are unlucky you get say 3 volts across the .025 ohms of your probes ground cable and the wire to the panel (remember the high quality ground at the ground point) = oh 120 amps and good by probe.

The scariest scenario is a short low current run at the end of a long feeder line. There is no breaker in the ground for obvious reasons , the small 16 or 18 gauge wire heats until it melts perhaps causing a fire. Now the designed ground is open, but safty exsists through the other ground path at least until the system is changed and the second ground is removed, This results in a serious safty condition.
Note that the temporary cause condition is often a piece of service equiptment like a welder or a pump in a plant setting, or a drill or other temporary load that has been serviced incorectly in the home setting.

The "fuse" action will happen at the first spot to exceed the melt temp of the wire - an insulated and thus often hidden point.

As the instalation ages and changing use requirements cause additions and deletions from the electrical system the ground system deviates from the design thus my comment on adding a backup generator to an older system .

Hope I haven't over done this but over the 30 plus years I have spent doing this grounding has been the least understood concept for all the people involved, and without doubt the root cause of most problems. regards Terry